Gastropod diversity dynamics at the Paleozoic–Mesozoic transition and the impact of the Carnian biotic crisis on gastropod diversity

During the transition from the Paleozoic to the Mesozoic, marine communities went through major changes in clade composition and proportions. A significant shift happened in the proportion between the Paleozoic Evolutionary Fauna and the Modern Evolutionary Fauna. The Permian-Triassic transition also marks the establishment of molluscs as the most diverse and dominant group in marine benthic environments. At the end-Permian, gastropods experienced a mass extinction as all other marine clades, but they showed a remarkable proliferation in the Triassic. As a result, the global species and genus diversity exceeded their maximum diversity during the Permian. However, not all gastropod clades contributed to the remarkable Triassic diversification. Similar to the shift in clade proportions between the Paleozoic and Modern Evolutionary Fauna, the clade proportions within Gastropoda changed at the Permian-Triassic transition as a result of differential diversification and selective extinction. The change in the proportional diversity of individual gastropod clades is most evident in the dwindling of the order Pleurotomariida (Vetigastropoda). Although Pleurotomariida was one of the most diverse and abundant Late Paleozoic gastropod groups, they diversified poorly during the Triassic recovery period compared to other groups. Gastropod diversity peaked in the Carnian (Late Triassic), but many newly evolved Carnian taxa could not pass the upper Carnian boundary, indicating an extinction event in the Carnian. A previous analysis of occurrence data in the Paleobiology Database (PBDB) indicates that among the marine clades, gastropods suffered most from the Carnian biotic crisis. Here, we analyze the generic range through data from the most comprehensive global Triassic gastropod list (2177 species, 429 genera) and the PBDB. The range-through data suggest high extinction rates in the Carnian (45%) compared to extinction rates during previous Triassic ages (5%). The Carnian extinction rates were higher than in the Rhaetian (35%). If singletons are excluded from the analysis, the Carnian still shows the highest extinction rate (16%) when compared with the Early and Middle Triassic ages (1–3%), but lower than in the Rhaetian (33%). Origination rates continuously decrease throughout the Triassic. Shareholder quorum subsampling analysis of the PBDB data indicates that generic extinction rates rose in the Carnian, peaked in the Norian and remained high in the Rhaetian. The analyses of both, range through and occurrence data show persistently high extinction rates throughout the Late Triassic. Altogether the Late Triassic biotic crises seem to impact gastropod diversity as much as the end-Permian mass extinction event.